1. Field of the Invention
The present invention relates to a balun used to connect a balanced transmission line to an unbalanced transmission line and a semiconductor device provided with the balun.
2. Description of the Prior Art
A balun that operates in a low frequency band and is used to connect a balanced transmission line to an unbalanced line consists of a concentrated constant component such as a transformer, whereas a balun that operates in a high-frequency microwave band consists of a distributed constant component. Since most of baluns each of which consists of a distributed constant component include a quarter-wavelength matching element or are transformers whose size is determined according to usable wavelengths, a disadvantage to them is that their frequency bands are fundamentally narrow.
FIG. 23 is a perspective view showing the structure of a prior art balun 100 which is in practical use and operates in a microwave band, the balun having small wavelength dependence and a large frequency band. In the figure, reference numeral 101 denotes a first conductive layer that is tapered, and reference numeral 102 denotes a second conductive layer that is tapered.
As shown in FIG. 23, the first conductive layer 101 is tapered from a maximum width at an end portion 101a thereof to a minimum width at another end portion 101b thereof, and the second conductive layer 102 is tapered from a maximum width at an end portion 102a thereof to a minimum width at another end portion 102b thereof. The taper of each of the first and second conductive layers 101 and 102 can be a linear taper. As an alternative, the taper of each of the first and second conductive layers 101 and 102 can be, as to shifting characteristic impedance, an exponential taper, a triangular taper, a Klopfenstein taper, or any other taper which can reduce the amount of reflection while transforming the characteristic impedance of a balanced transmission line into the characteristic impedance of an unbalanced transmission line over a large frequency band. Furthermore, in order to hold the spacing between the first and second conductive layers 101 and 102, they are usually formed on both sides of a dielectric substrate such as a printed board (not shown in the figure), respectively.
In operation, the balun 100 connects an unbalanced line coupled to the end portions 101a and 102a of the first and second conductive layers 101 and 102 to a balanced line coupled to the other end portions 101b and 102b, and also transforms the characteristic impedance of the unbalanced line to the characteristic impedance of the balanced line. In order to minimize the amount of reflection due to changes in the characteristic impedance of the balun, the taper of each of the first and second conductive layers 101 and 102 can be optimized.
By the way, it is possible to easily connect a coaxial connector to the end portions 101a and 102a on the unbalanced-line side of the balun 100 shown in FIG. 23 because the pair of the end portions 101a and 102a is a normal microstrip line, but since the end portions 101b and 102b on the balanced-line side of the balun are formed on the both sides of a substrate not shown in the figure, respectively, it is difficult to physically connect them to a pair of balanced terminals, which is formed on an electronic component, such as an IC, and which is arranged in the same plane of the substrate.
Therefore, in order to connect an electronic component, such as an IC, which has a pair of balanced output terminals in the same plane, to another electronic component having a pair of unbalanced input terminals, one of the pair of balanced output terminals is connected to a grounded surface of the substrate by way of a termination. On the other hand, the other one of the balanced output terminal pair is connected to one unbalanced input terminal of the other electronic component by way of a microstrip line.
FIG. 24 is a plan view showing the structure of a prior art balun 200 which can be incorporated into a power amplifier for use with television broadcasting transmitters as disclosed in Japanese patent application publication (TOKKAIHEI) No. 9-46106. FIG. 25 is a bottom view of the balun 200. In the figure, reference numeral 201 denotes a first conductive layer formed on a top surface of a printed board 220, and reference numeral 202 denotes a second conductive layer formed on a bottom surface of the printed board 220. These conductive layers 201 and 202 form a broadside-coupling-type line and constitute an isolation transformer 203. The first and second conductive layers 201 and 202, which constitute the isolation transformer 203, both have a predetermined width. Reference numeral 204 denotes a high-frequency signal input terminal to which an end of the first conductive layer 201 is connected, reference numeral 205 denotes an output terminal to which another end of the first conductive layer 201 is connected, reference numeral 206 denotes a output terminal to which the end of the second conductive layer 202 is electrically connected via a through hole 210, reference numerals 207a and 207b denote third and fourth conductive layers with a ground potential, reference numeral 208 denotes a fifth conductive strip layer that connects the first conductive layer 201 to the third conductive layer 207a and that functions as an inductance, and reference numeral 209 denotes a sixth conductive layer that is formed in the form of a strip and that connects the second conductive layer 202 to the fourth conductive layer 207b and functions as an inductance. A push-pull circuit transistor (not shown in the figure) for use in power amplifiers is connected to the pair of output terminals 205 and 206.
In the prior art balun constructed as shown in FIGS. 24 and 25, a high-frequency signal, which is applied to the high-frequency signal input terminal 204 by way of a microstrip line which is an unbalanced line, flows through the first and second conductive layers 201 and 202 which constitute the isolation transformer 203, as a pair of two equal-amplitude currents 180 degrees out of phase with each other. One of the electric current pair is supplied from the first conductive layer 201, by way of the output terminal 205, to one terminal of a push-pull circuit transistor (not shown in the figure) for use in power amplifiers, and the other one of the electric current pair is supplied from the second conductive layer 202, by way of the through hole 210 and the output terminal 206, to another terminal of the push-pull circuit transistor.
A problem with prior art baluns that operate in a microwave band constructed as above is that although conductive layers have to be tapered in order to provide small wavelength dependence and a large frequency band, it is difficult to make a physical connection between a balun including such conductive layers and an electronic circuit, such as an IC, having a pair of balanced terminals, as previously mentioned. Another problem is that when one of the pair of balanced terminals is connected to a ground by way of a termination, the efficiency is reduced because the other one of the pair of balanced outputs is not used, and the load on a differential circuit that generates a pair of balanced outputs becomes unbalanced because of an inductance included in the termination which increases with increasing frequency, which results in a malfunction of the differential circuit.
Furthermore, a problem with the prior art balun as disclosed in Japanese patent application publication No. 9-46106 shown in FIGS. 24 and 25 is that the pattern of the conductive layers is complex and the balun is not suitable for use with a connection of a balanced line with an unbalanced line over a wide frequency band requested by 40 Gbps optical communication.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a broadband balun with a simple structure that facilitates a connection of itself with a pair of balanced terminals of an electronic circuit disposed in the same plane of a substrate, and a semiconductor device provided with the balun.
In accordance with an aspect of the present invention, there is provided a balun comprising: a first conductive layer disposed on a top surface of a substrate, the first conductive layer having first and second end portions; a second conductive layer having a shorter length than the first conductive layer and disposed on the top surface of the substrate, the second conductive layer having first and second end portions, the first end portion of the second conductive layer serving as a balanced transmission line in cooperation with the first end portion of the first conductive layer; the substrate having a through hole electrically connected to the second end portion of the second conductive layer; and a third conductive layer disposed on a bottom surface of the substrate, the third conductive layer having a first end portion electrically connected to the second end portion of the second conductive layer via the through hole, and a second end portion that serves as an unbalanced transmission line in cooperation with the second end portion of the first conductive layer, and the third conductive layer being tapered from a maximum width at the second end portion thereof to a minimum width at the first end portion thereof. Accordingly, the balun can transform balanced mode into unbalanced mode over a wide frequency range. The balun can also facilitate the connection of itself with an electronic circuit, such as an IC, having a pair of balanced output terminals in the same plane of the substrate.
In accordance with another aspect of the present invention, the taper of the third conductive layer is a curved taper.
In accordance with a further aspect of the present invention, the taper of the third conductive layer is a Klopfenstein taper as to shifting characteristic impedance.
In accordance with another aspect of the present invention, the first end portion of the third conductive layer has a width smaller than that of the second end portion of the second conductive layer.
In accordance with a further aspect of the present invention, the third conductive layer includes a strip segment having a certain width smaller than that of the second end portion of the second conductive layer, and extending from the first end portion of the third conductive layer.
In accordance with another aspect of the present invention, the second end portion of the third conductive layer has a width that is at least from four to five times as large as that of the second end portion of the first conductive layer.
In accordance with a further aspect of the present invention, the second end portion of the third conductive layer has a width that is substantially equal to a diameter of an outer conductor of a coaxial cable to be electrically connected to the second end portion of the third conductive layer.
In accordance with another aspect of the present invention, the third conductive layer has a length equal to or greater than one-half of a wavelength of a microwave to be transmitted through the balun.
In accordance with a further aspect of the present invention, there is provided a balun comprising: a first conductive layer disposed on a top surface of a substrate, the first conductive layer having first and second end portions, and the first conductive layer being tapered from a maximum width at the second end portion thereof to a minimum width at the first end portion thereof; a second conductive layer having a shorter length than the first conductive layer and disposed on the top surface of the substrate, the second conductive layer having first and second end portions, the first end portion of the second conductive layer serving as a balanced transmission line in cooperation with the first end portion of the first conductive layer; the substrate having a through hole electrically connected to the second end portion of the second conductive layer; and a third conductive layer disposed on a bottom surface of the substrate, the third conductive layer having a first end portion electrically connected to the second end portion of the second conductive layer via the through hole, and a second end portion that serves as an unbalanced transmission line in cooperation with the second end portion of the first conductive layer.
In accordance with another aspect of the present invention, the taper of the first conductive layer is a curved taper.
In accordance with a further aspect of the present invention, there is provided a balun comprising: first and second substrates that are laminated; a first conductive layer disposed between the first and second substrates, the first conductive layer having first and second end portions; a second conductive layer having a shorter length than the first conductive layer and disposed between the first and second substrates, the second conductive layer having first and second end portions, the second end portion of the second conductive layer serving as a balanced transmission line in cooperation with the first end portion of the first conductive layer; the laminated first and second substrate having a through hole electrically connected to the second end portion of the second conductive layer; a third conductive layer disposed on a top surface of the laminated first and second substrates, the third conductive layer having a first end portion electrically connected to the second end portion of the second conductive layer via the through hole, and a second end portion that serves as an unbalanced triplate transmission line in cooperation with the second end portion of the first conductive layer, and the third conductive layer being tapered from a maximum width at the second end portion thereof to a minimum width at the first end portion thereof; and a fourth conductive layer disposed on a bottom surface of the laminated first and second substrates, the fourth conductive layer having a first end portion electrically connected to the second end portion of the second conductive layer via the through hole, and a second end portion that serves as the unbalanced triplate transmission line in cooperation with the second end portion of the first conductive layer and the second end portion of the third conductive layer, and the fourth conductive layer being tapered from a maximum width at the second end portion thereof to a minimum width at the first end portion thereof. Accordingly, the balun can transform balanced mode into unbalanced mode over a wide frequency range. The balun can also facilitate the connection of itself with an electronic circuit, such as an IC, having a pair of balanced output terminals in the same plane between the laminated substrates having a triplate structure.
In accordance with another aspect of the present invention, each of the tapers of the third and fourth conductive layers is a curved taper.
In accordance with a further aspect of the present invention, each of the first end portions of the third and fourth conductive layers has a width smaller than that of the second end portion of the second conductive layer.
In accordance with another aspect of the present invention, there is provided a semiconductor device comprising: an electronic circuit disposed on a top surface of a substrate, the circuit having a pair of balanced terminals; a balun formed on the substrate, for connecting the pair of balanced terminals to an unbalanced transmission line, the balun including a first conductive layer disposed on the top surface of the substrate, the first conductive layer having a first end portion connected to a terminal of the pair of balanced output terminals of the electronic circuit, and a second end portion, a second conductive layer having a shorter length than the first conductive layer and disposed on the top surface of the substrate, the second conductive layer having a first end portion connected to the other terminal of the pair of balanced output terminals of the electronic circuit, and a second end portion, the substrate having a through hole electrically connected to the second end portion of the second conductive layer, and a third conductive layer disposed on a bottom surface of the substrate, the third conductive layer having a first end portion electrically connected to the second end portion of the second conductive layer via the through hole, and a second end portion that serves as the unbalanced transmission line in cooperation with the second end portion of the first conductive layer, and the third conductive layer being tapered from a maximum width at the second end portion thereof to a minimum width at the first end portion thereof; and an electronic module mounted on the substrate, for transmitting or receiving a signal to or from the electronic circuit by way of the balun. Accordingly, the balun according to the present invention makes it possible to effectively use outputs from the pair of balanced output terminals of the electronic circuit without connecting one of the pair of balanced output terminals to a grounded surface of the substrate by way of a termination, thereby preventing the operating status of the electronic circuit from becoming unstable, and improving the efficiency of the semiconductor device.
In accordance with a further aspect of the present invention, the semiconductor device further comprises a coaxial cable for electrically connecting the balun to the electronic module.
In accordance with another aspect of the present invention, the electronic module is electrically insulated from a ground of the substrate.
In accordance with a further aspect of the present invention, the electronic module transmits or receives a high-frequency signal to or from the electronic circuit by way of the balun, and a high-frequency signal line of the electronic module is electrically insulated from the ground of the substrate.
In accordance with another aspect of the present invention, the electronic module is connected to the substrate by way of an insulating member.
In accordance with a further aspect of the present invention, the electronic module is an optical module driven by a pair of signals supplied, by way of the balun, from the electronic circuit.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.